Cold-drawn, straightened and stress relieved steel wire for prestressed concrete and method for production thereof

ABSTRACT

Cold-drawn, straightened and stress relieved high carbon steel wire for prestressed concrete, and a method for the production thereof, having reduced load loss due to stress relaxation and good ductility. The steel comprises, by weight percent, from about 0.70 percent to about 0.90 percent carbon, about 0.5 percent to about 1.0 percent manganese, about 0.025 percent maximum phosphorus, about 0.035 percent maximum sulfur, about 0.15 percent to about 0.35 percent silicon, about 0.010 percent to about 0.020 percent nitrogen, about 0.010 percent to about 0.030 percent columbium, and remainder essentially iron except for incidental impurities. The cold-drawn wire has a pearlitic microstructure wherein the pearlite colony size is reduced by adding columbium to confer good ductility, and sufficient uncombined nitrogen is present to provide improved stress relaxation properties. Processing includes austenitizing a hotreduced rod stock at about 980*C to about 1030*C and isothermal treating at about 540* to about 590*C, followed by air cooling. The stock is then cold-drawn into wire, straightened, and stress relieved.

United States Patent [1 1 Lorenzetti et al.

[4 1 Aug. 19, 1975 COLD-DRAWN, STRAIGHTENED AND STRESS RELIEVED STEELWIRE FOR PRESTRESSED CONCRETE AND METHOD FOR PRODUCTION THEREOF [75]Inventors: James J. Lorenzetti, Franklin; James N. Cordea, Monroe, bothof Ohio [73] Assignee: Armco Steel Corporation,

Middletown, Ohio [22] Filed: Aug. 27, 1974 21 Appl.No.:500,932

[52] U.S. Cl. 148/12 B; 148/36 [51] Int. Cl C21d 9/52 [58] Field ofSearch 148/12 B, 36

[56] References Cited UNITED STATES PATENTS 3,532,560 10/1970 Tomioka etal. 148/12 B 3,844,848 10/1974 Stacey 148/12 B Primary Examiner-W.Stallard Attorney, Agent, or Firm-Melville, Strasser, Foster & Hoffman[5 7] ABSTRACT Cold-drawn, straightened and stress relieved high carbonsteel wire for prestressed concrete, and a method for the productionthereof, having reduced load loss due to stress relaxation and goodductility. The steel comprises, by weight percent, from about 0.70percent to about 0.90 percent carbon, about 0.5 percent to about 110percent manganese, about 0.025 percent maximum phosphorus, about 0.035percent maximum sulfur, about 0.15 percent to about 0.35 percentsilicon, about 0.010 percent to about 0.020 percent nitrogen, about0.010 percent to about 0.030 percent columbium, and remainderessentially iron except for incidental impurities. The cold-drawn wirehas a pearlitic microstructure wherein the pearlite colony size isreduced by adding columbium to confer good ductility, and sufficientuncombined nitrogen is present to provide improved stress relaxationproperties. Processing includes austenitizing a hot-reduced rod stock atabout 980C to about 1030C and isothermal treating at about 540 to about590C, followed by air cooling. The stock is then cold-drawn into wire,straightened, and stress relieved.

7 Claims, N0 Drawings COLD-DRAWN, STRAIGHTENED AND STRESS RELIEVED STEELWIRE FOR PRESTRESSED CONCRETE AND METHOD FOR PRODUCTION THEREOFBACKGROUND OF THE INVENTION lished Dec. 14, 1971, discloses high carbonsteel wire having excellent resistance against relaxation by reason of atotal nitrogen content of 0.008 to 0.05 percent by weight (the steelfurther containing 0.55 to 0.85 percent carbon, 0.30 to 0.90 percentmanganese, 0.15 to 0.35 percent silicon and remainder iron except forimpurities unavoidable in the steel-making process) wherein aluminum isrestricted to a maximum of 0.015 percent by weight. Other nitrideformers such as titanium, vanadium, columbium and the like, should alsobe kept at a low level, according to the disclosure, in order to permitthe presence of at least about 0.008 percent by weight uncombinednitrogen, thereby attaining steel wire having reduced load loss due tostress relaxation. Silicon is-used as a deoxidant in order to obtain afully killed starting material.

United States Pat. No. 3,671,334, issued June 20, 1.972 to J. H. Bucheret a1, discloses a formed and strainaged steel article having a yieldstrength above 70 ksi, and a process for the production thereof, thesteel containing 0.08 to 0.18 percent carbon, 0.3 to 1.0 percentmanganese, 0.01 to 0.05 percent columbium, 0.008 to 0.014 percentnitrogen, 0.10 percent maximum silicon, less than a total of about 0.02percent of aluminum, zirconium, vanadium and titanium, and the balanceessentially iron. The steel of this reference is thus a low car- I grainsize of the steels. It. is pointed out that columbium is the only one ofthe conventionally employed grain refining elements, i.e., zirconium,vanadium and titanium, which is not also a strong nitride former; and,consequently, is the only one of these elements which can be used inconjuction with nitrogen to produce a strainaging steel.

An article by T. Gladman et al. in Journal of the Iron and SteelInstitute, pages 916-930 (December 1972) discusses the influence ofaluminum, titanium, vanadium and niobium (columbium) as grain refinersin steels containing 0.4 to 0.6 percent carbon, 1.4 to 1.5 percentmanganese, 0.3 to 0.9 percent silicon, 0.016 to 0.021 percent nitrogen,and balance essentially iron. From experimental data reported therein,it was concluded that less than the expected dispersion strengtheningwas obtained when useing niobium, and may be attributed to the veryrestricted solubility of niobium in i a high-carbon austenite. Thearticle was concerned with factors controlling yield and tensilestrengths and impact transition temperature in high carbonferritepearlite steels, and it was found that low uncombined or freenitrogen was necessary in order to obtain low impact transitiontemperatures. Hence, only aluminum, titanium and vanadium wereconsidered advantageous in steels having a high total nitrogen content.

While the above-mentioned Japanese Patent Publication disclosed highcarbon wire having reduced load loss due to stress relaxation, by reasonof a relatively high uncombined nitrogen content, this increase innitrogen would render the steel subject to excessive breakage duringwire drawing and cold straightening if heated above about 940C beforedrawing;

The problem of obtaining adequate ductility in prestressed wire of thetype disclosed in this Japanese reference is not believed to be obviousto a person skilled in the art in view of the disclosures of theabove-mentioned Bucher et al. patent or the Journal of the Iron andSteel Institute articles. The Japanese reference emphasizes thecriticality of maintaining the elements aluminum, titanium, vanadium andcolumbium at a low level. More specifically, the maximum aluminumcontent is stated to be 0.015 percent while the minimum total nitrogencontent is 0.008 percent.

The disclosure in the Bucher et al patent that columbium is not a strongnitride former would be of no benefit to a person skilled in the art iffaced with the problem of obtaining good ductility in a cold drawn wireof the type dislcosed in the Japanese reference. Bucher et al disclose arelatively low carbon steel having a fully ferritic microstructure inthe cold rolled and strained condition. The columbium addition in such asteel controls grain size after the austenite to ferrite transformation,a mechanism which is entirely different from that of transformation of ahigh carbon steel to pearlite.

In ferritic steels particles such as carbides in the grain boundariesrestrict the ferritic grain size. A fully pearlitic structure has grainboundaries which are not as mobile as ferrite boundaries. Hence thepearlite colony size, after transfon'nation, is controlled completely bythe grain size of the prior austenite. In contrast to this, in aferritic structure, or in a ferritic-pearlitic structure, ferrite grainsize is controlled only to a minor extent by particles in the prioraustenite phase and predominantly by particles suchas columbium carbidesafter transformation to ferrite.

Similar observations apply to the ferrite-pearlitic structures of the0.4 to 0.6 percent carbon steels of the above-mentioned Journal article,despite the unsupported statement at page 922 thereof that pearlitecolony size depends on the prior austenite grain size which in turn canbe controlled by grain-refining additions. However, in-the reported datathe addition of 0.05 percent columbium results in no refining of thepearlite colony size, apparently due to the cooling cycle followedtherein.

Moreover, in the Journal article columbium was added in an amount of0.05 percent. This columbium level forms a carbide which is stable up toabout 1370C in a steel containing 0.8 percent carbon and a nitride whichis stable up to about 1130C in a steel containing 0.012 percentnitrogen. Accordingly, the addition of 0.05 percent columbium to a highcarbon steel may result in precipitation of large carbides duringorimmediately after solidification of the cast ingot, and such carbidescannot be dissolved during subsequent processing since heating to atemperature up to 3 about 1370C cannot be practiced without adverseeffect on mechanical properties.

It is therefore evident that the prior art has not disclosed norsuggested the provision of cold-drawn, high carbon steel wire having afully pearlitic microstructure which combines reduced load loss due tostress relaxation with good ductility.

It is a principal object of the present invention to provide such aproduct and a method for the production thereof.

SUMMARY The present invention provides cold-drawn, high carbon steelwire for prestressed concrete having a fully pearlitic microstructure,reduced load loss due to stress relaxation and good ductility,consisting essentially of, by weight percent, from about 0.70 percent toabout 0.90 percent carbon, about 0.5 to about 1.0 percent manganese,about 0.025 percent maximum phosphorus, about 0.035 percent maximumsulfur, about 0.15 to about 0.35 percent silicon, about 0.010 to about0.020 percent total nitrogen, about 0.010 to about 0.030 columbium,substantially all the columbium being combined with carbon, andremainder essentially iron except for incidental impurities.

The method of the present invention comprises providing a hot reducedrod stock of the above composition, austenitizing the stock by heatingin the range of about 980 to about 1030C, transforming the stock to afully pearlitic microstructure by isothermal heating at about 540 toabout 590C, air cooling, whereby to obtain a pearlite colony sizeranging between about 15 and about 30p., and cold-drawing, straighteningand stress relieving the rod stock into wire for prestressed concrete.

The austenitizing and isothermal heat treatments of the process of theinvention are rapid (comprising a time of about minutes foraustenitizing and about 1 to 2 minutes for austenite to pearlitetransformation, these times being subject to minor variations dependingupon rod diameter), and hence the process provides an efficient and highproduction rate.

In the present invention the addition of from 0.010 to 0.030 percentcolumbium to a steel containing from about 0.70 to about 0.90 percentcarbon is critical in achieving improved ductility. More specifically,the hot reduced rod stock after heat treatment will exhibit tensilereduction-in-area values ranging from about 25 to about 35 percent. Byway of comparison, a steel having a similar composition withoutcolumbium addition exhibits tensile reduction-in-area of about 20 toabout 22 percent when subjected to the same heat treatment.

Cold-drawn steel wire for prestressed concrete produced in accordancewith the invention exhibits a load loss by the 1000 hours stressrelaxation test at 20C and 67.5 percent initial stress of not greaterthan about 3 percent. By comparison, cold-drawn steel wire containing nocolumbium and having a total nitrogen content not greater than about0.007 percent exhibits a load loss by the same test of about 6 percent.

DESCRIPTION OF THE PREFERRED EMBODIMENTS While the composition of thesteel has been set forth above in broad limits, optimum properties areobtained in a steel in which carbon is from about 0.80 to about I 0.85percent, manganese about 0.80 to about 0.90 percent, silicon about 0.20to about 0.30 percent, nitrogen from about 0.011 to about 0.016 percent,columbium from about 0.014 to about 0.020 percent, with the maximumphosphorus and sulfur contents being as set forth above, and remainderessentially iron except for incidental impurities. At least about 0.008percent uncombined nitrogen is preferably present in the final product.

Although not critical, preferably incidental impurities other thanphosphorus and sulfur are maintained within the following limits:

Copper 0.20% maximum Chromium 0.15% maximum Nickel 0.15% maximumMolybdenum 0.05% maximum The carbon, manganese, nitrogen and columbiumranges are critical, and departure therefrom results in loss of one ormore of the desirable properties.

A minimum of about 0.70 percent carbon and preferably about 0.80 percentis necessary in order to provide adequate strength and to insuretransformation to a fully pearlitic microstructure under the desiredheat treatment conditions. More than about 0.90 percent carbon wouldadversely affect the cold-drawing properties of the steel. Preferably amaximum of about 0.85 percent carbon is observed for optimum properties.

A minimum of about 0.5 percent manganese, preferably about 0.80 percent,is believed to be desired for its effect in holding nitrogen in solutionin the steel. However, a maximum of about 1.0 percent manganese andpreferably about 0.90 percent, must be observed in order to avoid undulylong austenite to pearlite transformation time during heat treatment.

About 0.010 percent minimum nitrogen is necessary in order to achievethe marked improvement in reduced load loss due to stress relaxationwhich is one of the principal objects of the present invention. Morethan about 0.020 percent total nitrogen is undesirable because of itstendency to produce extensive strain aging and consequent brittleness. Apreferred maximum of 0.016 percent is observed for this reason.

To obtain homogeneously sound steel it is necessary to add silicon as adeoxidizer during the steel making process in the range of about 0.15percent to about 0.35 percent silicon. Preferably aluminum is not addedsince it ties up free nitrogen.

A columbium range of about 0.010 percent to about 0.030 percent, andpreferably from about 0.014 percent to about 0.020 percent, isnecessary, at the carbon levels involved, in order to produce relativelysmall columbium carbide particles only after solidification of themolten steel in amounts which will concentrate in the austenite grainboundaries. Columbium in excess of about 0.030 percent results information of relatively massive carbides which are stable up totemperatures well above the austentizing heat treatment range of 980 to1030C of the present process. As indicated previously, fine columbiumcarbide particle size and distribution in the grain boundaries of theprior austenite grains are essential in the present invention forcontrol of pearlite colony size and resultant improved ductility. v

Heats of similar composition have been prepared with and withoutcolumbium additions, hot reduced to rod stock, austenitized at about980C, transferred to a lead bath at about 555C for controlledtransformation to pearlite, and air cooled. Pearlite colony size andASTM grain size are compared below for two such heats together withanalyses of the elements carbon, manganese, silicon, nitrogen andcolumbium:

ASTM Prior Pearlite Austenite Colony Example Composition Grain SizeSize-p.

C Mn Si N Cb A .82 .82 .24 .010 none 484 5 100-70 B .85 .83 .27 .012.014 8&9 25-18 load loss after the 1000 hour stress relaxation test at20C and 67.5 percent initial stress was about 2.8 percent for bothExamples, indicating no detrimental effect of columbium on load losscharacteristics. The reduced load loss and improved ductility of thecolumbium containing wire of the present invention are thereforesignificant.

In summary, the composition of the steel, and the heat treatment in themethod of the present invention are critical in achiveing thecombination of reduced load loss due to stress relaxation, goodductility and high strength.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. Cold-drawn, high carbon steel wire for prestressed concrete having afully pearlitic microstructure, reduced load loss due to stressrelaxation, and good ductility, said wire being cold-drawn from hotreduced rod stock having a tensile reduction-in-area value of at leashabout 25 percent after austenitizing and transformation to pearlite,said steel consisting essentially of, by weight percent, from about 0.70to about 0.90 percent carbon, about 0.5 to about 1.0 percent manganese,about 0.025 percent maximum phosphorus, about 0.035 percent maximumsulfur, about 0.15 to about 0.35 percent silicon, about 0.010 to about0.020percent total nitrogen, about 0.010 to about 0.030 percentcolumbium, substantially all the columbium being combined with carbon,and remainder essentially iron except for inciden tal impurities.

2. The wire claimed in claim 1, wherein carbon is from about 0.80 toabout 0.85 percent, manganese is from about 0.80 to about 0.90 percent,silicon is from about 0.20 to about 0.30 percent, nitrogen is from about0.011 to about 0.016 percent, and columbium is from about 0.014 to about0.020 percent.

3. The wire claimed in claim 1, wherein at least about 0.008 percentuncombined nitrogen is present.

4. A method of producing cold-drawn steel wire for prestressed concretehaving reduced load loss due to stress relaxation and good ductility,comprising the steps of providing a silicon killed, hot reduced rodstock consisting essentially of, by weight percent, from about 0.70 toabout 0.90 percent carbon, about 0.5 to about 1.0 percent manganese,about 0.025 percent maximum phosphorus, about 0.035 maximum sulfur,about 0.15 to about 0.35 percent silicon, about 0.010 to about 0.020percent total nitrogen, about 0.010 to about 0.030 percent columbium,and remainder iron except for incidental impurities, austenitizing saidstock by heating in the range of about 980 to about 1030C, transformingsaid stock to a fully pearlitic microstructure by isothermal heating toabout 540 to "about 590C, and air cooling, whereby to obtain a pearlitecolony size ranging between about 15 and 30p, and cold-drawing,straightening and stress relieving said rod stock into wire of desiredfinal diameter.

5. The method claimed in claim 4, wherein said rod stock contains fromabout 0.80 to about 0.85 percent carbon, about 0.80 to about 0.90percent manganese, about 0.20 to about 0.30 percent silicon, about 0.011

' to about 0.016 percent nitrogen, and about 0.014 persize.

1. Cold-drawn, high carbon steel wire for prestressed concrete having afully pearlitic microstructure, reduced load loss due to stressrelaxation, and good ductility, said wire being cold-drawn from hotreduced rod stock having a tensile reduction-in-area value of at leastabout 25 percent after austenitizing and transformation to pearlite,said steel consisting essentially of, by weight percent, from about 0.70to about 0.90 percent carbon, about 0.5 to about 1.0 percent manganese,about 0.025 percent maximum phosphorus, about 0.035 percent maximumsulfur, about 0.15 to about 0.35 percent silicon, about 0.010 to about0.020percent total nitrogen, about 0.010 to about 0.030 percentcolumbium, substantially all the columbium being combined with carbon,and remainder essentially iron except for incidental impurities.
 2. Thewire claimed in claim 1, wherein carbon is from about 0.80 to about 0.85percent, manganese is from about 0.80 to about 0.90 percent, silicon isfrom about 0.20 to about 0.30 percent, nitrogen is from about 0.011 toabout 0.016 percent, and columbium is from about 0.014 to about 0.020percent.
 3. The wire claimed in claim 1, wherein at least about 0.008percent uncombined nitrogen is present.
 4. A METHOD OF PRODUCINGCOLD-DRAWN STEELWIRE FOR PRESTRESSED CONCRETE HAVING REDUCED LOAD LOSSDUE TO STRESS RELAXATION AND GOOD DUCTILITY, COMPRISING THE STEPS OFPROVIDING A SILICON KILLED, HOT REDUCED ROD STOCK CONSISTING ESSENTIALLYOF, BY WEIGHT PERCENT, FROM ABOUT 0.70 TO ABOUT 0.90 PERCENT CARBON,ABOUT 0.5 TO ABOUT 1.0 PERCENT MANGANESE, ABOUT 0.025 PERCENT MAXIMUMPHOSPHORUS, ABOUT 0.035 MAXIMUM SULFUR, ABOUT 0.15 TO ABOUT 0.35 PERCENTSILICON, ABOUT 0.010 TO ABOUT 0.020 PERCENT TOTAL NITROGEN ABOUT 0.010TO ABOUT 0.030 PERCENT COLUMBIUM, AND REMAINDER IRON EXCEPT FORINCIDENTAL IMPURITIES, AUSTENITIZING SAID STOCK BY HEATING IN THE RANGEOF ABOUT 980* TO ABOUT 1030*C, TRANSFORMING SAID STOCK TO A FULLYPEARLITIC MICROSTRUCTURE BY ISOTHERMAL HEATING TO ABOUT 540* TO ABOUT590*C, AND AIR COOLING, WHEREBY TO OBTAIN A PEARLITE COLONY SIZE RANGINGBETWEEN ABOUT 15 AND 30U, AND COLD-DRAWING, STRAIGHTENING AND STRESSRELIEVING SAID ROD STOCK INTO WIRE OF DESIRED FINAL DIAMETER.
 5. Themethod claimed in claim 4, wherein said rod stock contains from about0.80 to about 0.85 percent carbon, about 0.80 to about 0.90 percentmanganese, about 0.20 to about 0.30 percent silicon, about 0.011 toabout 0.016 percent nitrogen, and about 0.014 percent to about 0.020percent columbium.
 6. The method claimed in claim 4, wherein saidprestressed wire contains at least about 0.008 percent uncombinednitrogen.
 7. The method claimed in claim 4, wherein the columbiumcontent is so selected as to result in formation of small columbiumcarbide particles which are distributed in the asutenite grainboundaries in the hot reduced rod stock, whereby to control theaustenite grain size.